1. Field of the Invention
The present invention relates to a copper foil for a printed circuit board, a method for fabricating the same, and a trivalent chromium conversion treatment solution used for fabricating the same, and more particularly, to a copper foil for a printed circuit board, a method for fabricating the same, and a trivalent chromium conversion treatment solution used for fabricating the same, which has an excellent stability of Zn and chromate plating film forming amounts.
2. Description of the Related Art
A copper foil or a copper alloy foil (hereinafter, referred to simply as “copper foil”) is widely used for the purpose of a conductor (conductive member or conductive strip). Particularly in the field of the flexible printed circuit (FPC), a printed circuit board is manufactured by layering (laminating) a copper foil on a polyimide film or by coating a copper foil with a varnish mainly composed of polyamic acid. Hereinafter, materials such as polyimide film, varnish, or solidified varnish to be used for the printed circuit board are referred as “base material (substrate) for a printed circuit board” or simply as “base material”.
A good bonding is required between the copper foil and the base material for a printed circuit board. Therefore, the roughening treatment is frequently conducted for an bonding surface of the copper foil to increase an anchoring effect, thereby improving the bonding with the base material for a printed circuit board.
The copper foil is classified into an electro-deposited copper foil and a rolled copper foil according to the manufacturing method therefor. However, the roughening treatment is conducted in similar manner for these two types of copper foils. For example, as a manner of roughening treatment, a manner of applying (depositing) copper in the form of rice-grains on a surface of the copper foil by burnt plating and a manner of selectively etching grain boundaries by using acid are generally used.
As to the roughening treatment using the burnt plating, the roughening treatment by alloy platings has been developed in addition to the usual copper plating. Japanese Patent Laid-Open (Kokai) No. 52-145769 (JP-A-52-145769) discloses copper-nickel alloy plating as a representative example of alloy platings.
In addition, as to a surface treatment after such a roughening treatment, Japanese Patent Publication for Opposition (Kokoku) No. 6-54829 (JP-B-6-54829) proposes to provide a cobalt plating or cobalt-nickel alloy plating, etc.
On one hand, as to a method for improving the bonding with the base material, there is a method for providing an anchoring effect by using the roughening treatment (improvement in physical bondability), as described before. Further, there is also a step for improving a chemical bondability between the copper foil and the base material for a printed circuit board by a surface treatment for providing a metallic layer with a high affinity with the base material on a surface of the copper foil.
Conversion treatment such as so-called chromate treatment or silane-coupling treatment conducted for a surface of the copper foil is an example of the method for providing the metallic layer with the high affinity with the base material on the copper foil surface. The chromate treatment and silane-coupling treatment are methods for the purpose of rust preventing the surface of the copper foil as well as for the purpose of improving the bondability with the base material for a printed circuit board, as disclosed in Japanese Patent No. 3142259 and Japanese Patent Laid-Open (Kokai) No. 2005-8972 (JP-A-2005-8972). In addition, the rust prevention (corrosion resistance, oxidation resistance) effect is expected (required) also for the surface of the copper foil, which is a surface not to be bonded with the base material (the back side of the bonding surface).
As disclosed in the JP-B-6-54829, Japanese Patent No. 3142259, and JP-A-2005-8972, the chromate treatment has been conducted by immersing the copper foil to be treated in a treatment solution containing hexavalent chromium, or by electrolyzing the copper foil which is provided as an anode or a cathode in the chromate treatment solution. However, considering the recent emphasis on the environmental protection, the conversion treatment free of any hexavalent chromium has been developed.
As an example of the conversion treatment free of any hexavalent chromium, the chromate treatment using a treatment solution containing trivalent chromium is best exploited and commercialized. In the present invention, such a treatment is indicated as “trivalent chromium conversion treatment”, or simply as “(trivalent) chromate treatment”. However, the treatment solution actually on the market has been developed as a treatment solution for the purpose of plating of the automotive parts. Therefore, a Zn film used as a underlying layer for the chromate treatment is thick (equal to or more than about 2 μm), and the chromate film is formed to be thick.
On the other hand, the treatment film forming amount required for the purpose of electronic parts, such as the copper foil for a FPC, is generally 3 to 5 μm/cm2 for Zn and 0.3 to 0.5 μm/cm2 for Cr (measured by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES), conversion unit of the chromate film is Cr). Converting the ranges of the film forming amount into ranges for film thickness, the total thickness of the Zn film and chromate film is several nanometers to several dozens nanometers.
The film forming amounts and film thicknesses of the Zn film and chromate film required for the purpose of the electronic parts are smaller (thinner) by 2 or 3 orders, compared with those for the purpose of the automotive parts. Therefore, it is not appropriate to use the chromate treatment solution for processing the automotive parts as the chromate treatment solution for processing the electronic parts.
The thickness of respective chromate treatment films formed on the copper foil for a printed circuit board is very thin. Therefore, it is difficult to observe or analyze the film thickness by an energy dispersive X-ray spectrometry or scanning electron microscope, so that the film thickness is often observed or analyzed by the ICP-AES measurement.
As an example for responding to the above requirements, Japanese Patent Laid-Open No. 2005-42139 (JP-A-2005-42139) discloses a treatment solution containing trivalent chromium for plating the electronic parts. In JP-A-2005-42139, a chromate treatment solution which is an aqueous solution free of hexavalent chromium ion, but containing 1.4 mg/L or more and less than 125 mg/L of trivalent chromium ions, 0.8 mg/L or more and less than 40 mg/L of fluorine ions, 2.5 mg/L or more and less than 70 mg/L of nitric acid is disclosed.
However, according to the chromate treatment using a conventional and commercially available treatment solution containing the trivalent chromium, the zinc (Zn)-plating layer may dissolve (elute) in mass. Therefore, it is necessary to form the zinc galvanized layer thick for adjusting a Zn film forming amount to a required value. If the Zn film forming amount is small (i.e. the film thickness is thin), the controllability of the Zn film forming amount (namely, film thickness) will be deteriorated (i.e. it is difficult to control the Zn film forming amount). Further, since the Zn film forming amount affects the chromate film forming amount, the controllability of the chromate film forming amount (namely, film thickness) will be deteriorated. As a result, an excessively formed chromate film may give adverse effects to the bonding with the base material such as polyimide, etching controllability, Sn-plating solution resistance, etc.
In addition, according to the chromate treatment using the treatment solution containing the trivalent chromium disclosed in JP-A-2005-42139, the zinc galvanized film and chromate treatment film can be formed with a predetermined film thickness of nanometer order. However, the improvement in controllability of Zn film forming amount and chromate film forming amount is not described.